The invention relates to single ended radiant tubes used for furnace heating. The conventional design for this purpose has at least three concentric tubular parts, the outermost or first of which contains the burning fuel and provides the radiant heating effect. The flow of combustion gases in this tube is opposite in direction to that of flow in the other tubes. The innermost of the tubes supplies the fuel to be burnt up to a burner head on the end of that tube and located between the two ends of the outermost tube. Combustion air is supplied through an intermediate tube surrounding the fuel tube and effectively inside the flow of combustion products contained in the outer tube. This is so that the air supplied may cool the burner parts behind the flame and so that the air is pre-heated.
The burner comprises a number of ports for the fuel flow, which especially in small diameter tubes necessarily open radially in order to get good fuel air mixing. This creates the risk of flame impingement on local spots on the surrounding tube, which can lead to premature burn-out. In fact, premature failure of these tubes is almost invariably caused by burn-out due to local overheating.
It is usual to provide a fourth tube lying within the air supply tube so as to confine the combustion air to a smaller cross-sectional area and achieve better heat transfer. This also gives a more rapid flow of the air past the burner so as to carry the flame and combustion products along the tube in an attempt to avoid the radial hot spots adjacent the burner as mentioned. This improvement is particularly difficult to achieve satisfactorily in smaller diameter radiant tubes.
In prior Patent GB 2 133 527 the fourth tube terminates short of the burner head so as to expose the final part of the second tube (fuel supply) to the combustion air, and the head itself has spiral vanes which are intended to induce a vortex flow system in the combustion air with the aim of confining the burner flame to the central part or axis, further to prevent flame impingement on the surrounding tube. This feature is of course only effective over the length of gas flow in the same direction as that of fuel and air up to the burner, and ceases to be effective in the reverse flow portion.